A Lesson on DatingGeolog

ic

How old is old?

How do you date a rock?

• Absolute Dating– Analogous to your

age . . . i.e. an exact number

– Often found using the radioactive decay of certain elements

• Relative Dating– Dating with regard to

an order of events . . . analogous to knowing that you are the middle child

– Found by examining the local geology and discovering what happened before and after

Who would want to date a rock?

Three simple rules for dating rocks:

• Law of Original Horizontality– i.e. if you dump out a bunch of sand, it won’t stack

up vertically . . .

• Law of Cross-Cutting Relationships– In order to cut something, something has to be

there before you cut it

• Law of Superposition– If you are making a sandwich, you can’t put the

top on first . . .